(Reuters) -Bluebird bio and Carlyle Group ( CG ) and SK Capital Partners said on Wednesday that the private equity firms have offered $5 per share upfront to takeover the gene therapy maker.

Shareholders may elect to receive either the new offer or the previous offer of $3 per share, with an additional $6.84 contingent upon bluebird achieving certain sales milestones.

(Reporting by Puyaan Singh in Bengaluru' Editing by Anil D'Silva)